Method of production of a spark plug for internal combustion engines

ABSTRACT

A method of producing a spark plug for internal combustion engines comprising arranging a metal piece formed of a noble material on an end face of at least one metal electrode of the spark plug and directing laser beams onto an outer surface of the metal piece facing the spark gap to form an alloy zone between the one metal electrode and a wear-resistant layer facing the spark gap, the alloy zone consisting of materials of the metal electrode and the metal piece with the proportion of the material of the metal electrode continuously decreasing from an area abutting the metal electrode and consisting only of the metal electrode material, to an area abutting the wear-resistant layer which consists only of the material of the metal piece.

BACKGROUND OF THE INVENTION

The invention relates to a spark plug for internal combustion engines. Aspark plug is already known (U.S. Pat. No. 4,540,910) in which anintermediate layer is arranged between the metal electrode, whichconsists of a nickel alloy, and a highly wear-resistant metal coatingconsisting of a platinum containing alloy, which intermediate layerserves to compensate for the sharply diverging heat expansion behaviorof the metal electrode and the metal coating; this intermediate layerconsists of an alloy which is composed of a platinum alloy and nickel.In order to apply the wear-resistant metal coating to the metalelectrode, the wear-resistant metal coating is first mechanically platedtogether with the intermediate layer and the intermediate layer,provided with the metal coating is then connected with the metalelectrode by resistance welding.

A spark plug in which a coating of noble or precious metal such asplatinum is attached to the free end face of the center electrode byresistance welding has also already been described in the DE-PS No. 3132 814. In this center electrode, however, there is the problem that thenoble metal layer becomes detached from the center electrode because ofstresses in the connecting region during higher thermal and corrosiveloads.

The DE-PS No. 22 56 823 shows a spark plug with a center electrode whoseend face is protected against high wear by a platinum piece. Thisplatinum piece is provided with an intermediate layer plated thereon ina similar manner to the example according to the U.S. Pat. No. 4,540,910cited above; this intermediate layer comprises a material whichpossesses the same or approximately the same high resistance totemperature and corrosion and thermal expansion characteristic as thecenter electrode itself. This intermediate layer consists of anickel-base alloy of which the center electrode itself can also becomposed. This intermediate layer is connected with the center electrodeby welding, particularly by resistance welding. When using such sparkplugs in internal combustion engines, it has also been found out thatthe fastening of the platinum pieces on the electrodes is notsatisfactory.

SUMMARY OF THE INVENTION

The invention has the object of developing a method of producing a sparkplug in which an connection of the metal electrode and wear-resistantmetal coating is achieved, and the production of which can be carriedout at a low cost.

This object is achieved by arranging a metal piece formed of a noblematerial on the gap facing end face of an electrode and directing laserbeams onto the metal piece to form an alloy zone between the end face ofthe electrode and a wear-resistant layer, which alloy zone consists ofthe materials of the electrode and the metal piece with the proportionof the electrode material continuously decreasing from an area adjacentto the end face of the electrode which includes only electrode material,to an area adjacent the wear-resistant layer which includes only thematerial of the metal piece connecting the metal electrode andwear-resistant metal coating is very simple because it requires only asingle metal piece without a second metal piece to be plated thereon toprovide the wear-resistant metal coating on the metal electrode.Moreover, the process is suitable for large-series production withoutdifficulties and ensures that the wear-resistant metal coatings arrangedon the metal electrodes are maintained with a long working life in alloccurring operating conditions in the internal combustion engine.

These and other objects and advantages of the present invention will bebest understood from the following description of the preferredembodiment with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an enlarged partial longitudinal cross-sectional view ofthe area of a spark plug on the combustion chamber side;

FIG. 2 shows a partial enlarged longitudinal cross-sectional view, ofthe center electrode on the combustion chamber side, according to FIG.1, (preliminary stage: the metal piece not yet connected with centerelectrode); and

FIG. 3 shows a view similar to the view according to FIG. 2 with themetal piece connected with center electrode via alloy layer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The portion of a spark plug 10 on the ignition side, which is shown inFIG. 1, has an adjustment attachment portion on the connection sidewhich substantially corresponds to that of the spark plug according toDE-OS No. 22 45 404 which corresponds to U.S. Pat. No. 3,909,459. Theend portion of the metal housing on the ignition side is designated by areference numeral 11 and the longitudinal bore in this metal housing 11has a reference numeral 12. The insulating body 13 the end face 14 ofwhich on the combustion chamber side extends substantially flush withthe end face 15 of the metal housing 11 on the combustion chamber side,is arranged within this longitudinal bore 12 of the metal housing 11.The insulating body 13 is provided on its circumferential surface withan annular pump space 16 which is arranged substantially adjacent to theend face 14 of the insulating body 13 and communicates with thecombustion chamber of the internal combustion engine via an annular gap17, the combustion chamber not being shown in detail. The pump space 16ensures that no conductive bridge of deposits can form in the plane ofthe end face 14 of the insulating body 13 between the insulating body 13and the metal housing 11. Keeping open of the annular gap 17 resultsfrom expanding of a fuel vapor-air mixture (not shown) located in thepump space 16 at the moment of ignition which mixture blows through theannular gap 17 and accordingly eliminates any deposits. The insulatingbody 13 comprises a longitudinal bore 18 in which a first metalelectrode 19 is displaced and, on the combustion chamber side, projectsabove the end face 14 of the insulating body 13; but this metalelectrode 19 can also be flush with the end face 14 of the insulatingbody 13, or can possibly also project further out of the longitudinalbore 18 of the insulating body 13. This first metal electrode 19commonly extends coaxially with the longitudinal axis of the spark plug10 and is generally designated as a center electrode. The free endportion of the first metal electrode 19 facing toward the combustionchamber is provided with a metal coating 20 consisting of a materialhaving high resistance to wear (noble metal) which preferably consistsof platinum or a platinum alloy. This first metal electrode 19 which isprovided with a metal coating 20 is located opposite the free endportion of a second metal electrode 22 and is spaced therefrom formingthe so-called air gap 21. The free end portion of the second electrodeis preferably likewise provided with a metal coating 20A of a highlywear-resistant material. This second metal electrode 22 is usuallyfastened at the end portion of the metal housing 11 on the combustionchamber side (e.g. by welding) and is electrically grounded andaccordingly constitutes the so-called ground electrode. In the sparkplug 10 shown in FIG. 1, the second metal electrode 22 is constructed ina hook-like manner and has a smaller cross section in the area of itsmetal coating 20A than in the area where it is connected with the metalhousing 11. As a result of this design of the second metal electrode 22,the heat absorbed by this second metal electrode 22 during the operationof the spark plug 10 is quickly transmitted to the metal housing 11,specifically to the area which also carries a thread 11A and whichrapidly transmits the heat to the engine block. However, the inventionrelating to the metal coatings 20, 20A is not limited to a spark plug 10described above, which also possesses an air gap 21 as well as acombined surface-air gap (14, 17, 11) at the same time; rather it issuitable for all spark plugs which comprise at least one metalelectrode.

By way of the example of the first metal electrode 19, the so-calledcenter electrode, FIGS. 2 and 3 show the process according to which themetal coating 20 consisting of noble metal is applied to the end face 23on the combustion chamber side; in the present example, platinum is usedas highly wear-resistant material. In the present example, and alsoconventionally, the center electrode 19 consists of a nickel alloy andhas a diameter d in the range of 0.8-2.5 mm, but preferably between 1and 1.3 mm. The end face 23 directed toward the spark gap 21 is coveredwith a metal piece 24 which consists of the highly wear-resistantmaterial, that is, platinum or a platinum alloy, and has a thickness sof 0.3 mm; the thickness s of this metal piece 24 is between 0.2 and 0.5mm, but preferably between 0.25 and 0.35 mm. The metal piece 24 has adiameter which substantially corresponds to the diameter d of the centerelectrode 19; depending on the case of application, however, thediameter of this metal piece 24 can also be slightly smaller or alsosomewhat greater than the diameter d of the center electrode 19. Theupper side of the metal piece 24 facing the air gap 21 is designated bya reference numeral 25. However, instead of such a disk-shaped metalpiece 24, a drop-shaped, spherical or cap-shaped metal piece can also bearranged on the end face 23, or possibly also on an end face 23 when itis roughened, grooved or provided with one or more recesses (not shown);a flat cup-shaped recess is particularly suitable for reducing shearingstresses. Laser beams L are then directed on the upper side 25 of themetal piece 24 in such a way that they run substantially parallel to theimaginary center line M of the first metal electrode 19 and aredimensioned in such a way that an alloy zone 27 is formed in the area ofthe underside 26 of the metal piece 24 and the end face 23 of the centerelectrode 19. However, this alloy zone 27 does not extend to thewear-resistant layer 28 consisting of platinum or a platinum alloy whichfaces toward the spark gap. As a result of this connection process, thisalloy zone 27 is formed in such a way that a layer forming area 29 whichfaces the metal electrode 19 consists solely of the material of thecenter electrode 19, that is, of a nickel alloy, and the proportion ofmaterial of this center electrode 19 within the alloy zone 27 has atendency to become continuously smaller in the direction of its layersforming area 30 facing the wear-resistant layer 28, so that there is nolonger any material of the center electrode 19 contained in thelayer-shaped area 30 facing the wear-resistant layer 28. The thicknessof such an alloy zone 27 is between 50 and 200 μm, preferably between100 and 150 μm. In the preferred embodiment, the alloy zone 27 extendsinto a recess 31 which extends into the center electrode 19 in a conicalor cup-shaped manner; the deepest point of this recess 31 lies in theregion of the center line M of the center electrode 19.

A particularly secure connection between the metal coating 20 and themetal electrode 19 can be achieved when the center electrode 19 makes arotational movement R around its center line M during the connectionprocess; if the metal electrode 19 is rotated around its center line Mduring the application of the process, it is also possible to direct thelaser beams L diagonally on the upper side 25 of the metal piece 24 andaccordingly to obtain a more favorable alloy zone 27. Pulse laser beamsare preferably applied in this process; but oscillating laser beams mayalso be used. Because of this form of an alloy zone 27, the divergingexpansion behavior of the material of the metal electrode 19 and themetal coating 20 is compensated for, and the metal coating 20 isconsequently prevented from falling off the metal electrode 19. In somecases of application, it is also advantageous if the employed metalpiece 24 has a greater diameter than the metal electrode 19, the outersurface area adjoining the end face 23 of the metal electrode 19 canalso be constructed, if necessary, in the manner of a truncated cone oras a step, so that the metal coating then extends along a predeterminedarea of the outer surface area of the metal electrode 19.

The above statements regarding the metal coating 20 apply in acorresponding manner to the metal coating 20A on the second metalelectrode 22 and also to metal electrodes and multiple-electrodesdesigned in a different manner.

While the invention has been illustrated and described as embodied in amethod of producing spark plugs for internal combustion engines, it isnot intended to be limited to the details shown, since variousmodifications and structural changes may be made without departing inany way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentiallycharacteristics of the generic or specific aspects of this invention.

We claim:
 1. A method of producing a spark plug for internal combustionengines and comprising at least two metal electrodes located oppositeone another in a spaced relationship and forming a spark gaptherebetween with at least one metal electrode having a metal coating ofa high wear-resistant noble material on an end face thereof facing thespark gap and an intermediate layer containing materials of the metalcoating and the metal electrode, said method comprising the stepsof:arranging a metal piece formed of a noble metal on the spark gapfacing end face of the at least one metal electrode; and directing laserbeams onto an outer surface of the metal piece facing the spark gap toform an alloy zone between the metal electrode and a wear-resistantlayer facing the spark gap, and consisting of materials of the metalelectrode and the metal piece, and comprising a first layer-formed areaabutting the metal electrode and consisting only of a material of themetal electrode, and a second layer-formed area abutting thewear-resistant layer and consisting of a material of the metal piece, aproportion of the material of the center electrode in the alloy zonedecreasing continuously from the first layer-formed area to the secondlayer-formed area.
 2. A method according to claim 1, further comprisingthe step of rotating the one metal electrode about an axis extendingperpendicular to the spark gap facing end face thereof when directingthe laser beam onto the metal piece arranged on the spark gap facing endface of the one metal electrode.
 3. A method according to claim 1,wherein said laser beam directing step includes aligning the laser beamssubstantially perpendicular to the spark gap facing end face of the onemetal electrode.
 4. A method according to claim 1, further comprisingthe step of forming a recess in the spark gap facing end face of the onemetal electrode into which the alloy zone extends.
 5. A method accordingto claim 4, wherein the recess forming step includes forming the recessin an area of an axis of the one metal electrode.
 6. A method accordingto claim 1, further comprising the step of providing the spark gapfacing end face of the one metal electrode with a diameter in a rangefrom 0.8 mm to 2.5 mm.
 7. A method according to claim 6, wherein thediameter providing step includes providing the spark gap facing end faceof the one metal electrode with a diameter in a range from 1 mm to 1.3mm.
 8. A method according to claim 7, further comprising the step ofproviding the metal piece having a thickness in a range from 0.2 mm to0.5 mm.
 9. A method according to claim 8, wherein the step of providingthe metal piece includes providing a metal piece having a thickness in arange from 0.25 mm to 0.35 mm.
 10. A method according to claim 8,comprising the step of forming the one metal electrode as a groundelectrode having a hook-like shape and a cross-section which increasesfrom its face end face facing the spark gap to an end face at which theground electrode is fastened to a housing of the spark plug.
 11. Amethod according to claim 1, comprising the step of providing the sparkgap facing end face of the one metal electrode with a recess prior toarranging a metal piece thereon.
 12. A method according to claim 11,wherein said recess providing step includes providing a cup-shapedrecess.